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1 A soil vapor extraction and air sparging (SVE/AS) system with two air sparging wells (SWI and • SW2) was installed in May 1994 The SVE/AS system operated from May 1994 to November ' 1996 and recovered petroleum hydrocarbon vapors from wells MW2, MW3, MW5, and MW6 (screened from 10 to 30 bgs), wells V1 and V2 (screened from 20 to 40 feet bgs), and well V4 (screened from 25 to 40 feet bgs) A total of approximately 14,500 pounds of petroleum hydrocarbons was removed from the vadose zone by the SVE/AS system (EA 1997) The mass of TPH-g removed over time is presented in tabular form in Appendix D, and graphically in Figure 7 As presented in Figure 7, the SVE influent concentration and the cumulative estimated ' pounds of TPH-g removed over time become asymptotic over time Also presented on Figure 7 is the average groundwater elevation in the vapor extraction wells and well construction details As of October 1995, approximately 14,000 pounds of the total 14,500 had been removed The groundwater elevation had risen approximately 2 5 feet by October 1995, indicating that the decline in influent concentrations and the associated pounds removed is attributable to the success of the VES rather than rising groundwater conditions which can submerge the screened portion of the vapor extraction system Based on previous site investigations (EA 1987a, 1987b, 1988a, 1988b, 1989a, 1989b, 1990, 1991b), petroleum hydrocarbons occur at the site between ' 17 and 42 feet bgs The performance of the SVE/AS system is documented in the quarterly groundwater monitoring reports In November 1996, a vacuum vaporizer well system, known as UVB (L7nderdruck Verdampfer Brunnen), was installed for groundwater remediation at the site The installation of the UVB system consisted of overdnlling existing groundwater recovery well RW3 to install the dual- screen UVB well for groundwater extraction-infection, and the installation of adjacent monitoring wells UVB-MW1 and UVB-MW2 to momtor system performance (EA 1997) This remedial option was selected as it prevented migration from an upgradient offsite source, while remediating the onsite groundwater hydrocarbon concentrations UVB technology was designed and developed by IE GmbH of Germany as a method for the in situ treatment of groundwater, and was first introduced in the United States in 1992 The WB system is an in situ groundwater remediation system which removes petroleum hydrocarbons using a combination of physical and biological processes The WB system creates a circulation cell that transports the mobile, dissolved-phase and residual-phase hydrocarbons to a central well + casing for treatment The treatment methodology is primarily air stripping, and secondarily bioremediation for the light and middle range hydrocarbon fractions Between 6 February 1997 and 22 September 2000, a total of 1,890 2 pounds of petroleum hydrocarbons as gasoline (TPH-g) was removed from groundwater by the WB system based on laboratory analytical results, and 8215 pounds based on field measurements (ETIC 2000b) Figure 8 presents the pounds removed based on both lab and field data over time The lower value of 821 5 pounds is viewed as more representative of the actual pounds removed by this remedial system as the field values are consistent and not dependent on a single high value, as are the laboratory data The operational measurements of the UVB system showed that it had reached the asymptotic stage of operation The WB system was therefore shut down as referenced in ETIC's letter dated 29 October 2000 (ETIC 2000c) The shutdown of the UVB • system was approved by the SJCPHS in a letter received in December 2000 (Appendix A) G%ProJects1739421MA5TER\W?P Ra08011tert2 doc 4